1. Technical Field
This invention relates to flowmeters, and more particularly to small line size flowmeters such as sanitary vortex flowmeters.
2. Background Information
Vortex flowmeters are commonly installed in-line within a process fluid flow conduit, to measure the flow rate of the process fluid. A bluff body in the fluid flow generates eddies, or vortices, downstream of the bluff body, on alternating sides of the bluff body. This trail of vortices is known as the Karman vortex street. The bluff body is commonly known as a shedder.
The meter is factory-calibrated, establishing the relationship between frequency and velocity, known as the K factor. The velocity of process flow may be determined from the frequency of the vortex shedding. This vortex shedding frequency may be captured by a sensor placed on the shedder, where it may efficiently detect pressure variations associated with the shedding.
Vortex flowmeters are used in a wide variety of processes, ranging from industrial applications such as oil and chemical processing, to sanitary applications such as food and pharmaceutical processing. While these flowmeters operate in a similar manner, regardless of the particular application, sanitary applications present challenges that typically are not found in other non-sanitary environments. For example, vortex flowmeters intended for sanitary applications must be capable of being thoroughly cleaned in order to meet applicable clean-in-place (CIP) requirements. In this regard, it is important to ensure that the flowmeters do not have crevices or pockets within which food particles, etc., may become trapped or lodged, and thus inhibit or prevent thorough cleaning between process runs. The U.S. Food and Drug Administration has even promulgated standards, such as the Pasteurization Milk Ordinance (PMO), 3A Sanitary Standards, in order to address these concerns. These standards have, however, proven to be relatively difficult to meet. Indeed, it is believed that to date, Invensys Systems, Inc., the assignee of the present invention, is the only major manufacturer which has qualified vortex flowmeters to these standards.
These Invensys vortex flowmeters have been able to comply with the PMO 3A Sanitary Standards by effectively embedding the sensor within the shedder. This configuration places the sensor sufficiently close to the vortex street to enable the single device to detect the pressure variations generated on both sides of the shedder during operation. This embedded location of the sensor also eliminates the need for a separate process penetration for the sensor, to thus eliminate the crevices or pockets that may be occasioned thereby.
While such embedded placement has been shown to effectively enable vortex flowmeters to meet these sanitary requirements, a drawback is that the shedder, and the corresponding inner diameter of the conduit, must be large enough to accommodate the sensor. This means that placing a sensor within the shedder may not be suitable for smaller line size conduits, e.g., those smaller than about 2 inches (5 cm) in diameter, because the shedder/integral sensor would block an inordinately large percentage of the cross-sectional area of the flowmeter conduit. In this regard, the vortex shedder typically has a precise geometry relative to the diameter of the flowtube. The width of the shedder face and the corresponding length and other dimensions of the shedder tail are all proportioned to the meter bore (inside diameter). In order to sense the vortex pressure pulses, a sensor diaphragm of a particular minimum size (effective area) is required to produce a signal from the sensor. Since the vortex sensor is integral with the shedder tail in a conventional sanitary vortex meter, the tail on sizes smaller than 2 inches has been found to not be large enough to accommodate an integral sensor. This factor has proven to be particularly problematic, since many food processing and other sanitary applications use line sizes of 2 inches (5 cm) or less.
Thus, a need exists for a sanitary vortex flowmeter, which efficiently measures vortex shedding in small line size conduits.